Dr. James Madara, executive vice president of AMA, penned a Wednesday letter to the heads of Amazon, Facebook, Google, Pinterest, Twitter and YouTube. He said the proliferation of vaccine misinformation online risks undermining the sound science behind vaccines and could spark the spread of easily preventable diseases.

"With public health on the line and with social media serving as a leading source of information for the American people, we urge you to do your part to ensure that users have access to scientifically valid information on vaccinations, so they can make informed decisions about their families' health," Madara wrote.

Madara added that the AMA has seen a decrease in vaccination rates, which threatens to "erase many years of progress as nearly eliminated and preventable diseases return."

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Tech companies say they're addressing the issue

Google said in a statement that it believes information discouraging parents from vaccinating their children is "concerning."

"We've put a lot of effort into curbing misinformation in our products -- from better Search ranking algorithms, to improving our ability to surface authoritative content, to tougher policies against monetization of harmful or dangerous content on YouTube," Google said.

Facebook did not immediately respond to CNN's request for comment but said on March 7 that it is working to reduce the distribution of vaccine misinformation on the social media platform. Facebook said it plans to reduce the rankings of anti-vaccination groups and pages and reject anti-vaccination ads.

Amazon has received AMA's letter and is reviewing it, a spokesperson confirmed. The world's largest online marketplace offers anti-vaccination content, including books and movies.

Pinterest announced last month that it had blocked all vaccine searches from its platform. Pinterest and Twitter did not immediately respond to requests for comment.

Vaccinations are crucial to public health

The head of England's National Health Service warned this month that anti-vaccination "fake news" on social media has spurred an uptick in measles cases and a decline in vaccination coverage.

In addition, the World Health Organization (WHO) has deemed vaccine hesitancy one of the biggest threats to global health in 2019.

"Vaccination is one of the most cost-effective ways of avoiding disease -- it currently prevents 2-3 million deaths a year, and a further 1.5 million could be avoided if global coverage of vaccinations improved," the WHO said.